Box-and-pin-type threaded joint having different pitches and pitch diameters



Feb. 26, 1963 H 'r. VAN DER WISSEL 3,079,181

PE TH DED JOINT HAVING DIFFERENT Box-AND-PIN-Ts'r PITCH DIAMETERSPITCHES Original Filed May 3, 1956 4 Sheets-Sheet l FIGURE 2 FIGURE lFIGURE 3 INVENTOR: H. Tm in HM HIS AGENT Feb. 26, 1963 H. -r. VAN DERWISSEL 3,079,181

BOX-AND-PIN-TYPE THREADED JOINT HAVING DIFFERENT I PI 3 AND PIT OriginalFiled May 3, 1956 TCHE CH DIAMETERS 4 Sheets-Sheet 2 FIGURE 4 FIGURE 5lNVENTOR: 6 TM 4 w BY: 9.H' 4

Hls AGENT Fell 1963 H. T. VAN DER WISSEL 3,079,181

BOX-ANDPINTYPE THREADED JOINT HAVING DIFFERENT PITCHES AND PITCHDIAMETERS Original Filed May a, 1956 4 Sheets-Sheet s 93 v ma \1 FIGURE8 INVENTOR: H Tm BY: 6. Hm

HIS AGENT Feb. 26, 1963 H. 1-. VAN DER WISSEL 3,079,131

BOX-AND-PIN-TYPE THREADED JOINT HAVING DIFFERENT PITCH DIA PITCHES ANDMETERS Original Filed May 3, 1956 4 Sheets-Sheet 4 FIGURE 9 FIGURE IOFIGURE ll INVENTOR= H Tm HIS AGENT United States atent Ofiice 3,079,181Patented Feb. 26, 1963 BOX-AND-PlN-TYPE THREADED JOINT HAVING DIFFERENTPITCHES AND PITCH DIAMETERS Herman Tobias van der Wissel, The Hague,Netherlands,

assignor to Shell Oil Company, a corporation of Delaware Originalapplication May 3, 1956, Ser. No. 582,357. Divided and this applicationMay 14, 1959, Ser. No. 818,241

Claims priority, application Netherlands Mar. 16, 1954 1 Claim. (Cl.285333) The invention relates to a box-and-pin-type threaded jointparticularly suitable in well drilling operations where it can be usedfor instance as a tool joint, such as a sucker-rod, casing collar, sub,drill collar or drill bit joint.

This application is a division of application Serial No. 582,357, filedMay 3, 1956, and now abandoned, which is in, turn, acontinuation-in-part of application Serial No. 494,479, filed March 15,1955, and now abandoned.

In this type of joints one of the members to be jointed (the pin)consists of a cylindrical or tapering threaded body, the other member(the box) having l3. threaded cylindrical or tapering inner surface.

The type of joint recommended by the American Petroleum Institute(A.P.I.) is most generally used. Theoretically, in this joint the taperof the tapering member of the pin is the same as that of the taperingmember of the box, and the pitch of the two screw threads is the same.The so-called A.P.I. screw thread has the following drawback:

When screwing the pin into the box in the theoretical case indicatedabove, screw threads of the pin entirely mate the grooves of the boxand, conversely, the space available between the screw threads of thepin is wholly taken up by the box screw threads, indicating that bothflanks of each screw thread of the pin as well as of the box arecompressed. Thus, a screw thread is wedged into each groove and notchingis therefore very liable to occur in the roots of the grooves. Thesituation becomes still worse when the A.P.I. screw thread is used injoints which have to transmit torque and/or are subjected to bendingstress. As will be explained hereinafter, with reference to shoulderedconnections, when discussing FIG. 1, in these known types of joints veryunfavorable stress concentrations occur near the roots of the grooveswhich are already subjected to heavy strains, so that tearing of thethread and rupture of the pin is not an uncommon feature.

In order to obviate these drawbacks and thus enable a joint to takeheavier loads, its dimensions being unchanged, according to theinvention either or both screw threads of the joint deviate in such away from the normal tapering screw thread that, when the pin has beenscrewed tight into the box, the outer flanks of the pin thread and theinner flanks of the box thread are more heavily loaded than the otherflanks. Thus, the threads are substantially equally loaded along theentire thread contact length Without plastic deformation, when aconnection is screwed power-tight. In the event that both threads of thejoint deviate, it is to be understood that they do not deviate in thesame way or to the same degree.

By the outer flanks of a screw thread are meant the flanks turned awayfrom the middle of the thread, i.e., the thread surfaces sloping downfrom the crest of a thread. By normal tapering screw thread is meant ascrew thread of which the taper and pitch are the same over the entirelength. I

The above feature can be obtained in various ways, for instance, byvarying the pitch near the middle in either or both screw threads, or bydesigning the screw thread of the small-diameter end of a tapering pinand the corresponding part of a tapering box with a pitch which is lessthan that of the screw thread of the largediameter end of the pin andthe corresponding part of the box. It is also possible to select for thepitch of the pin thread 'a larger value than for the box thread and tomake the difference between the pitch diameters of pin and box threadsmaller near the middle thereof than at either end. In the latter casethe taper of thebox along the entire length of the thread may beconstant, the taper of the pin at the large-diameter end being smallerand at the thin end. being larger than that of the box.

These change in the pitch of the screw threads or pitch diameters ensurethat the box grips around the pin in such a way that, when looking atthe joint placed in a horizontal position, only the left flanks of theleft half of the screw thread provided on, the pin and only theright-hand flanks of the right-hand half of this screw thread are loadedby the screw thread flanks of the box. A wedging effect between thethreads and the grooves, with all its detrimental consequences, is thusavoided. The engagement between the threads of the'pinand box in thismanner puts the pin and the box under compres sion and tension,respectively, the resulting material stresses comparing favorably withthose occurring with the construction previously discussed so thatgreater loads are permitted, provided the parts are properly designed.

Shrink-type joints are known in which the thread pitch of the box, whencold, is smaller than that of the pin, so that after the box has beenheated and allowed to shrink on the pin, In this known joint, the twoscrew threads do not deviate from a normal tapering screw thread whichresults in the terminal parts of the screw thread being much' moreheavily loaded than the more central parts. 'In this joint the outerflanks of the screw thread of the pin 'and the inner flanks of the screwthread of the box are not more heavily loaded than the other flanks,sothere is still a wedging eflect.

The invention is described in more detail with reference to thediagrammatical drawing, in which FIG. 1 indicates the known A.P.I.threaded joint, FIG. 2 detail A shown in FIG. 1, FIG. 3 a cross sectionof a joint according to the invention, FIGURES 4, 5, 6 and 7 are variousarrangements of detail B shown in FIG. 3, and FIGURES 8, 9, 10 and 11show the threaded joint according to the invention as used in varioustypes of joints. Where necessary, the actual proportions have beenexaggerated in the drawing for clearness of understanding.

The objet of the figures and their description is only to give someembodiments of the invention; it is to be understood that theapplication of the invention is not the latter is subjected tocompression.

limited to the joints shown in the drawing, but that it may also coverall the threaded joints used in well drilling operations, such as tooljoints, casing collars, subs, drill collar joints, drill bit joints, andsucker rod joints. These joints may also be of the shrink type ornon-shrink type and may be formed on rods or tubular elements.

The screw thread shown in FIG. 1 is part of the longitudinal section ofa joint consisting of a tapering pin member 1 provided with screw thread2, and a box member 3 having a tapering inside wall provided with screwthread 4. The dimensions of these screw threads are based on A.P.I.specifications. The taper of the box is usually slightly less than thatof the pin so as to prevent premature'jamming of the small end of thepin with resultant rupture at that place.

Pin and box should be screwed into each other to such an extent that thecompressive force between the shoulders and 6 of the pin and boxrespectively, and the wedging eifect between the screw threads issufiiciently large to transfer the torque to be transmitted by thejoint. FIGURE 1 shows the relative position of screw threads2 and 4-when the shoulders 5 and 6 already contact each other.

In FIG. 1 the first three threads of the screw thread are shownas'engaged, the others as still disengaged. The pin is screwed down intothe box to such an extent that the compression of shoulders 5 and 6together with the wedging effect of the mating threads provides theforce required for transmitting the torque. In the roots of thesegrooves a wedging efiect will easily occur. 'FIG. 2 shows the forceswhich screw thread 4 exercises on screw thread 2 (detail A in FIG. 1).With this stress pattern, part 7 of box 3 is subjected to compression,but part 8 of pin 1 is put under tension so that every condition issatisfied for causing rupture of the end of the pin at a specificstress. A fracture 9 is started from the root of one of the first turnsof screw thread 2.

According to the present invention, the threaded joint is so constructedas to prevent the occurrence of a wedging effect between the screwthreads when screwing the pin into the box, and to permit that a greatpart of the power to be applied in tightening the joint is used beforethe shoulders come into contact, in other Words a great part of thefriction and deformation occurs before the shoulders come into contact.

To this end, in the: embodiment according to FIG. 4 (detail B shown inFIG. 3) a local pitch variation is provided near the middle of the screwthread 12 on the pin 11 and near the middle of the screw thread 14 onthe box 13, the pitch of-the pin thread at this particular point beinglarger than" the normal pitch p, and the pitch of the box thread at thisparticular point being smaller than the normal pitch p. This ensuresthat the outer flanks of the pin screw thread 12 press against the innerflanks of the box screw thread 14, and that as a result thereof the partof the pin 11 which is provided with' thread is, apart from radialstresses, subjected to an axial compression stress, Whereas an axialtensile stress is set up in the corresponding part of the box 13.

Part 18 of the pin and part 17 of the box are not subjected to axialstress when the shoulders 15 and 16 are not in contact. When, however,the shoulders are loaded (for sealing or torque transmission), theseparts (18 and 17) are subjected to axial stress. These shoulder forces,of course, do cause tensile stress in the pin, but not near the rootsofthe grooves of the thread, as-the material there is compressed by theflank pressure, so that these places are safe-guarded against wedging.Consequently the joint according to the invention can be subjected tomuch higher loads than the conventional conical threaded joint. Further,as according to the invention the flanks over the whole thread length ofthe joint take parts of the load, thestrengthof the joint can be furtherincreased by increasing the thread length ofthe joint.

Although in FIG. 4 local pitch variations are shown in the pin screwthread as well as in the box screw thread, it is also possible andgenerally preferable for a pitch variation to be provided in only one ofthe engaging screw threads in such a way that the pin is subjected to anaxial compression stress and the box to an axial tensile stress. In theembodiment shown in FIG. 5 the thin end of the pin 21 is provided with ascrew thread 22 having the pitch 12 the thick end of the pin beingprovided with the screw thread 22 having the pitch p These screw threadsengage with the screw thread 24 and 24 of the box 23 which have thepitch p and respectively. By making pitch smaller than pitch p thecondition shown in FIG. 5 will be obtained when pin 21 is screwed intobox 23. As in FIG. 4 the pin is subjected to an axial compressionstress, and the box is simultaneously subjected to an axial tensilestress When the pin is screwed into the box.

The manner in which the various pitch variations are obtained is notessential. The screw thread to be provided with a variation can beinterrupted at that particular point, as at 29, or continue, it beingunderstood that the variation should be divided over a suflicient lengthso as to ensure that the stresses caused by tightening of the connectionare as uniform as possible on all contacting thread surfaces.

The choice of the flank angles of the screw thread also governs themagnitude and direction of the forces K acting on these flanks; the sizeof these angles should be adapted to practical requirements.

In the threaded joint shown in FIG. 6, the screw thread 32 of the pin 31has the pitch 12 The screw thread 34 of box 33 is designed with a pitchp By designing these screw threads (the pitch 17 being larger than thepitch p;) in such a way that the difference between the pitch diameters36 and of pin and box thread near the center of the screw threads issmaller than at either end, this design ensures abutment of the outerflanks of the pin' screw thread against the inner flanks of the boxscrew thread when screwing the pin into the box. Owing to thecompression during screwing resulting from this contact, the pin issubjected to an axial compression stress and the box axial tensilestress in this type of joint.

The embodiment shown in FIG. 7 is a modification of the type shown inFIG. 6. Here also the selected pitch p -of the pin screw thread isgreater than pitch p of the box screw thread. Starting from the boxpitch line V 45, which instead of being curved, such as line 35 in 'FIG.

6, is chosen straight, the taper of the screw thread 44 arranged in thebox will remain constant throughout the length of the screw thread. Inorder to ensure that the ;outer flanks of the pin screw thread areforced against the inner flanks of the box screw thread when screwingthe pin into the box, the pin screw thread should be divided into twoparts 42 and 42' having tapers which differ from each other and alsofrom the taper of the box screw thread so that the difference betweenthe pitch diameters of pin and box screw thread near the center thereofis smaller than at either end thereof.

To ensure a uniform loading of the flanks, whereby galling is prevented,the transition between the two tapering parts of the pin at theparticular point is designed to take place more or less graduallyinstead of abruptly (see zone 48 in FIG. '7); in that case the pitchlines of pin and box are, even in the middle, far enough apart toprevent galling.

It is also possible to keep the taper of the pin constant throughout thelength of the pin and construct the inside of the box with two partswith diiferent tapers in such a way that the taper of the part situatednear the box end is larger and the taper of the part situated near theinside of the box is smaller than that of the pin. Also in this case thepitch lines of pin and box should be kept far enough apart. p

The pitch p of the pin screw thread can be made to vary for the twoparts of the pin; it may, for instance, for

the left part of the pin, be equal to the box screw thread pitch p (FIG.7).

In the screw-type joints according to the invention, the flank slitsbetween the inner flanks of the pin and the outer flanks of the box instressless state (i.e., in hand-tight position, that is before the pinis screwed tight into the box) in general increase in size from themiddle of the threaded joint to either end, while the flank slitsbetween the outer flanks of the pin and the inner flanks of the boxdecrease in size from the middle of the threaded joint towards eitherend. It will be understood that the constructional features of thethreaded joints shown in FIGURES 6 and 7 can be combined with each otherand with those of the joints shown in FIGURES 4 and 5 to obtain thedesired result. Threaded joints according to the invention are indicatedgenerally by 96 in FIGURES 3, 8, 9, and 11.

A sealing as obtained in FIG. 3 by the shoulders 5 and 6 arranged atright anges to the axis of the joint, is obtained in FIG. 8 by thetapering faces 53 and 54.

Good results were also obtained in coupling the threaded joint 96 (seeFIG. 9) by preheating the entire box 63. After the pin has been screwedinto the box, the latter is allowed to cool. Owing to the decrease inlength of the box which is cooling down the pressure between theengaging flanks of'the two screw threads will increase, thus setting up,apart from radial stresses, an axial tensile stress in the box and anaxial compression stress in the pin, the screw thread beingsubstantially equally loaded at all parts. In most shrink-type jointsthere are radial stresses only, except in case of a joint where thethread pitch of the box is less than that of the pin resulting in theouter grooves being much more heavily loaded than the grooves situatedmore towards the middle.

With the shrink-type joint according to the invention the sealingaccording to FIG. 8 may be used with advantage.

Both the pin and box may consist of two abutting tapering parts 71 and73 (see FIG. 10), the taper of the part near the end being larger thanthat of the other parts situated on the pin. The latter taper may alsobe nil (see FIG. 11), in which case the pin comprises two parts, acylindrical part 81 and a tapering part 82 abutting thereto, the insideof the box 83 being also partly cylindrical and partly tapering. Thefirst type of construction has the advantage that the pin can be screwedmore rapidly into the box, whereas the construction with the cylindricalpart enables the use of a thicker pin.

If the screw thread is interrupted at the place of the pitch variation,fewer turns will be required for screwing down; before the pin isscrewed into the box it is inserted farther into the box than when thescrew threads are not interrupted. This construction may also beadvantageous in cutting the thread.

As a result of the stress distribution obtained according to theinvention in the box and pin, wobbling of the joint is prevented.

When using the screw thread according to the invention in casing, thiscasing can be pulled out of the formation without the risk of beingdisconnected on account of the screw threads being drawn over eachother.

While shoulders 5 and 6 in FIGURES 4 to 7 have been shown with a spacebetween them, they may be in contact with each other, e.g. to provideadditional sealing or to be stronger against bending stresses.

Test Results A tapering pin-and-box-type threaded joint according toA.P.I. specification was compared with a similar joint according to theinvention. Both joints were made from the same quality of steel, thetreatment of the steel having been the same in both cases. In both teststhe same grease was used and the two joints were tested in the samemanner with respect to loading conditions and temperature.

Differences between the two threads appear from the following table.

The two joints were gradually loaded (overloaded) with 3340 kgm., whichresulted in the following permanent changes of dimensions:

Normal Joint ae- A.P.I. cording to joint the invention Increase ofoutside diameter of shoulder of box mm 8. 05 1. 24 Increase of pitch ofpin mm 0. 66-0. 78 0. 05-0. 06 Decrease of inside diameter of pin at 50m.

from small end mm 0.92 0. 43

(The increase in pitch of the pin according to the invention was due tothe extreme overloading condition.)

A joint according to the invention with a thread length of 5 (1" largerthan the standard length), which resulted in a smaller shoulder area,was loaded (overload-ed) with 4000 kgm. The increase in outside diameterof the shoulder of the box was then 3.69 mm, but the increase in pitchof the pin was found to be between 0.005 (decrease) and +0.0l5 mm.,which is less than the tolerance of the thread.

Further a great number of bending tests have been made with smallmodels. In these tests it was found that the joint according to theinvention could be loaded by about 50% more than the A.P.I. joint,whereas, at the same load, the joint according to the invention couldstand a much larger number of loadings than the standard joint.

I claim as my invention:

In a screw-threaded joint for equipment comprising elongated cylindricalmembers to be joined together coaxially, a box having a taperedscrew-threaded portion formed on one of said members, a pin having atapered screw-threaded portion formed on another of said members, thescrew threads on said box and those on said pin being of substantiallyuniform cross section throughout their length, said screw-threadedportions being formed with the pitch of the thread on the pin largerthan the pitch of the thread in the box and with the difference betweenthe pitch diameters of the screw threads near the middle of pin and boxscrew-threaded portions being smaller than the differences at either endthereof, so that in a longitudinal cross-sectional View of the jointwhen the tightly coupled joint is in a horizontal position, with thelarger-diameter portion of the pin at the left, the left flanks of thethreads on the larger-diameter portion of the tapered pin and the rightflanks of the threads on the small-diameter portion of the tapered pinengage the screw-thread flanks of the box with significantly greaterforce than the other flanks of the pin, the difference in pitchdiameters being such that substantially all of the respective left andright flanks are in loaded engagement and whereby the threaded portionof the pin is subjected to an axial compressive force and the adjacentportion of the box to an axial tensile stress.

(References on following page) 7 References Cited in the file of thispatent 2,360,826 UNITED STATES PATENTS 2,870,668

1,817,808 Eaton Aug. .4, 1931 Eaton May 16, 5 2,345,725 Bettis Apr. 4,1944 Cherry Oct. 24, 1944 Flahout Jan 27, 1959 FOREIGN PATENTS GreatBritain May 30, 1945

